I've programmed ATMs, and dendory has it exactly right: those are completely regular stock PC hardware running a completely regular copy of Windows. The only thing special about them is that they're physically secured and have some very unusual peripherals and assorted drivers.

Honestly I never did program on an ATM so I may be wrong. But I'm willing to assume that unusual peripherals and drivers are analogous to the portability issues across android phones. Genuinely asking, not claiming to have the truth.

There's actually a standard for ATM hardware interfaces originally developed by Microsoft (WOSA/XFS, now CEN/XFS), which is a major reason why ATMs run Windows.

When I worked in the industry 10 years ago, there were lots of ambiguities in the standard and the drivers were pretty flaky, so the situation there may actually have looked like Android does now.

But ATM-specific hardware and software is a niche market with few customers paying lots of money each, so the pressure to eliminate incompatibilities is not as strong - you can just throw people and money at fixing issues as they crop up.

The situation is completely different with mass-market PC hardware and software though, and I'd say that it should be possible for Android to get there as well, since it's also a mass-market product.

Actually why would you think a Windows app wouldn't run on that ATM? I'd bet most would. It's probably a normal PC, with all the normal hardware. It would be slow, because it's most likely a slow one with no GPU acceleration and such, but I wouldn't be surprised if 95%+ of Windows apps ran on it. The only reason you, yourself couldn't test it is because they (obviously) run their own ATM software full screen and block you from installing stuff. But why wouldn't you think an ATM admin couldn't fire up Solitaire on it?